There are two basic methods for recording sound and music—analog and digital. See e.g. Ken C. Pohlmann, “The Compact Disc”, THE COMPUTER MUSIC & DIGITAL AUDIO SERIES, Volume 5. The above-mentioned audio series, which was published by A-R Editions, Inc., in Madison, Wis., is, along with all volumes therein, incorporated by reference.
In analog recording, the recording medium (a tape) varies continuously according to the sound signal. In other words, an analog tape stores sound signals as a continuous stream of magnetism. The magnetism, which may have any value within a limited range, varies by the same amount as the sound signal voltage.
In digital recording, the sound signal is sampled electronically and recorded as a rapid sequence of separately coded measurements. In other words, a digital recording comprises rapid measurements of a sound signal in the form of on-off binary codes represented by ones and zeros. In this digital system, zeros are represented by indentations or pits in a disc surface, and ones are represented by unpitted surfaces or land reflections of the disc, such that a compact disc contains a spiral track of binary codes in the form of sequences of minute pits produced by a laser beam.
Music that is input to a digital recording and the requisite series of reproduction processes, must pass through the recording side of a pulse code modulation (PCM) system. A master recording of the music is stored in digital form on a magnetic tape or optical disc. Once the magnetic tape has been recorded, mixed and edited, it is ready for reproduction as a CD. The CD manufacturer then converts the master tape to a master disc, which is replicated to produce a desired number of CDs. At the end of the PCM system is the reproduction side, the CD player, which outputs the pre-recorded music.
If digital technology is used in all intermediate steps between the recording and reproduction sides of the PCM system, music remains in binary code throughout the entire chain; music is converted to binary code when it enters the recording studio, and stays in binary code until it is converted back to analog form when it leaves the CD player and is audible to a listener. In most CD players, digital outputs therefrom preserve data in its original form until the data reaches the power amplifier, and the identical audio information that recorded in the studio is thereby preserved on the disc.
Optical Storage
The physical specifications for a compact disc system are shown in Prior Art FIG. 1. They were developed jointly by Sony and Philips, and are defined in the standards document entitled Red Book, which is incorporated herein by reference. The CD standard is also contained in the International Electrotechnical Commission standard entitled, Compact Disc Digital Audio System, also incorporated herein by reference. Disc manufacturers, as well as CD player manufacturers, obtain a CD license to use these specifications.
All disc dimensions, including those pertaining to pit and physical formations, which encode data, are defined in the CD standard. For example, specifications information on sampling frequency, quantization word length, data rate, error correction code, and modulation scheme are all defined in the standard. Properties of the optical system that reads data from the disc using a leaser beam are also defined in the standard. Moreover, basis specifications relevant to CD player design is located in the signal format specifications.
Referring to Prior Art FIGS. 2A and 2B, the physical characteristics of the compact disc surface structure are described. Each CD is less than 5 inches in diameter whose track thickness is essentially thinner than a hair and whose track length averages approximately 3 and a half miles. The innermost portion of the disc is a hole, with a diameter of 15 mm, that does not hold data. The hole provides a clamping area for the CD player to hold the CD firmly to the spindle motor shaft.
Data is recorded on a surface area of the disc that is 35.5 mm wide. A lead-in area rings the innermost data area, and a lead-out area rings the outermost area. Both lead-in and lead-out areas contain non-audio data used to control the CD player. Generally, a change in appearance in the reflective data surface of a disc marks the end of musical information.
A transparent plastic substrate comprises most of the CD's 1.2 mm thickness. Viewing a magnified portion of the CD surface, as shown in Prior Art FIG. 2A and 2B, the top surface of the CD is covered with a very thin metal layer of generally aluminum, silver or gold. Data is physically contained in pits impressed along the CD's top surface. Above this metalized pit surface and disc substrate lies another thin protective lacquer coating (10 to 30 micrometers). An identifying label (5 micrometers) is printed on top of the lacquer coating.
A system of mirrors and lenses sends a beam of laser light to read the data. A laser beam is applied to the underside of a CD and passes through the transparent substrate and back again. The beam is focused on the metalized data surface that is sandwiched or embedded inside the disc. As the disc rotates, the laser beam moves across the disc from the center to the edge. This beam produces on-off code signals that are converted into, for example, a stereo electric signal.
The Pit Track
Prior Art FIG. 3 shows a typical compact disc pit surface. Each CD contains a track of pits arranged in a continuous spiral that runs from the inner circumference to the outer edge. The starting point begins at the inner circumference because, in some manufacturing processes, tracks at the outer diameter of a CD is more generally prone to manufacturing defects. Therefore, CDs with shorter playing time provide a greater manufacturing yield, which has led to adoption of smaller diameter discs (such as 8 cm CD-3 discs) or larger diameter discs (such as 20 and 30 cm CD-Video discs).
Prior Art FIG. 4 shows a diagram of a typical track pitch. The distance between successive tracks is 1.6 micrometers. That adds up to approximately 600 tracks per millimeter. There are 22,188 revolutions across a disc's entire signal surface of 35.5 millimeters. Hence, a pit track may contain 3 billion pits. Because CDs are constructed in a diffraction-limited manner—creating the smallest formations of the wave nature of light—track pitch acts as a diffraction grating; namely, by producing a rainbow of colors. In fact, CD pits are among the smallest of all manufactured formations.
The linear dimensions of each track on a CD is the same, from the beginning of a spiral to the end. Consequently, each CD must rotate with constant linear velocity (CLV), a condition whereby uniform relative velocity is maintained between the CD and the pickup.
To accomplish this, the rotational speed of a CD varies depending on the position of the pickup. The disc rotates at a playing speed which varies from 500 revolutions per minute at the center, where the track starts, to 200 revolutions per minute at the edge. This difference in speed is accounted for by the number of tracks at each position.
For example, because each outer track revolution contains more pits than each inner track revolution, the CD must be slowed down as it plays in order to maintain a constant rate of data. So, when the pickup is reading the inner circumference of the CD, the disc rotates at the higher speed of 500 rpm. And as the pickup moves outwardly towards the disc's edge, the rotational speed gradually decreases to 200 rpm. Thus, a constant linear velocity is maintained, such that all of the pits are read at the same speed. The CD player constantly reads from synchronization words from the data and adjusts the disc speed to keep the data rate constant.
A CD's constant linear velocity (CLV) system is significantly different from an LP's system. A major difference stems from the fact that a turntable's motor rotates at a constant velocity rate of 33⅓ grooves. This translates into outer grooves having a greater apparent velocity than inner grooves, probably explained by the occurrence that high-frequency responses of inner grooves is inferior to that of outer grooves. If a CD used constant angular velocity (CAV) as opposed to the CLV system, pits on the outside diameter would have to be longer than pits on the inner diameter of the disc. This latter scenario would result in decreased data density and decreased playing time of a CD.
Like constant linear velocity, light beam modulation is also important to the optical read-out system that decodes the tracks. See Prior Art FIG. 5. A brief theoretical discussion on the distinctions between pit and land light travel explains this point.
Generally, when light passes from one medium to another with a different index of refraction, the light bends and its wavelength changes. The velocity at which light passes is important, because when velocity is slow, the beam bends and focusing occurs. Owing to several factors, such as the refractive index, disc thickness and laser lens aperture, the laser beam's size on the disc surface is approximately 800 μm. However, the laser beam is focused to approximately 1.7 μm at the pit surface. In other words, the laser beam is focused to a point that is a little larger than a pit width. This condition minimizes the effects of dust or scratches on the CD's outer surface, because the size of dust particles or scratches are effectively reduced along with the laser beam. Any obstruction less than 0.5 ml are essentially insignificant and causes no error in the readout.
As previously noted, a CD's entire pit surface is metalized. In addition, the reflective flat surface between each pit, (i.e. a land), causes almost 90 percent of laser light to be reflected back into the pickup. Looking at a spiral track from a laser's perspective on the underside of a disc, as shown in Prior Art FIG. 5, pits appears as bumps. The height of each bump is generally between 0.11 and 0.13 μm, such that this dimension is smaller than the laser beam's wavelength (780 nanometers) in air. The dimension of the laser beam's wavelength in air is larger than the laser's wavelength (500 nanometers) inside the disc substrate, with a refractive index of 1.55. In short, the height of each bump is, therefore, one-quarter of the laser's wavelength in the substrate.
Scientifically, this means that light striking a land will travel twice as far than light striking a bump. This discrepancy in light travel distances serve to modulate the intensity of a light beam. This allows data physically encoded on the disc to be recoverable by the laser.
Also, the pits and intervening reflective lands on the disc's surface do not directly designate ones and zeros. Rather, it is each pit's edge, whether leading or trailing, that is a 1 and all areas in between, whether inside or outside a pit, that are designated as zeros. Still, each pit and reflective land lengths vary incrementally. The combinations of 9 different pit and land lengths of varying dimensions physically encode the data.
Presently, there are three principal types of optical storage media for which there may be a need to provide security for the data stored on the optical storage media. The first type is a read-only memory (ROM) media where the disc is manufactured with the information already stored thereon in the form of depressions formed in the polycarbonate substrate. Read-only discs include CD-audio, CD-ROM, CD-interactive and CD-video discs.
The second type of optical storage media is a writable optical storage disc, which has the capability of having information recorded (or written) thereon after fabrication of the media.
And the third type is a re-writable or erasable optical storage disc, which has the capability of having information erased or modified after fabrication of the media.
In general, it is desired that the disc containing information is provided with a security marking or marker that is permanent, unalterable without damaging the disc medium, and could be determined by and/or related to the marking. The following prior patents represent the state of the art.
U.S. Pat. No. 4,961,077 to Wilson et al., incorporated herein by reference, discloses a method of affixing information characters on read-only optical discs by means of a pulsed scanning laser beam, which transmits light in a patterned array through a transparent layer of the medium, and indelibly marks the reflective layer of the medium without disrupting the surface continuity of the substrate and protective layer.
U.S. Pat. No. 5,625,816 to Burdick et al., incorporated herein by reference, discloses a method and system for tracking a manufactured product or group of manufactured products through a manufacturing process comprising a series of manufacturing steps performed at different physical locations.
U.S. Pat. No. 5,671,202 to Brownstein et al. relates to a method for providing security for the data stored in the optical information storage and retrieval system. The increased system security is provided by the inclusion in the medium of a visible and indelible identifying code and the storage of related data files on the storage medium. The visible identifying code is used in conjunction with the related files by the apparatus accessing the data files to protect the data files stored on the media against unauthorized access to the data files and/or unauthorized copying of the data files.
In U.S. Pat. No. 5,706,047 to Lentz et al., incorporated herein by reference, the invention relates generally to media upon which information is stored in an optical information storage and retrieval unit, and more particularly, to the inclusion in the media of an indelible identifying code embedded therein.
U.S. Pat. No. 5,706,266 to Brownstein et al., incorporated herein by reference, relates to a writable optical storage disc used in an optical information storage and retrieval system to provide security for the data stored in the storage and retrieval system.
The problem in one or more of the prior art references, I have determined, is that identification markings have been applied to the surface of the disc by means of mechanical disruption of the surface or by deposition of legible material on the surface. This information, however, being on the disc's surface can be compromised either accidentally or intentionally.
An additional problem in one or more of the prior art references is that the marking process is too sensitive to the energy level of the laser beam, such that too small an energy in the laser beam will not provide an identifiable marking, and too much energy can disrupt the lacquer overcoat layer and/or the polycarbonate layer used to protect the reflective layer.
Yet another problem in one or more of the prior art references is that the affixing or identifying marking information is easily applied to exterior, non-information surfaces of the substrate or protective layer, such as by printing. However, because the labeling or patterns are on the surface of the disc, they are susceptible to damage, alteration and can be removed too easily.
Accordingly, I have determined that it is desirable to solve one or more of the above problems. For example, I have determined that it is desirable to provide a system and method where identification markings need not be applied to the surface of the disc by means of mechanical disruption of the surface or by deposition of legible material on the surface.
I have also determined that it is desirable to provide a marking process that is not significantly sensitive to the energy level of the laser beam, and that will not disrupt the lacquer overcoat layer and/or the polycarbonate layer used to protect the reflective layer.
It is also desirable to provide a marking mechanism and/or process where the labeling or marking of the disc is not susceptible to damage, alteration, detection and/or removal.